Relative importance of geometrical and intrinsic water transport properties of active layers in the water permeability of polyamide thin-film composite membranes

Analytical and numerical modeling studies in the literature have shown that the water permeability of reverse osmosis (RO) membranes is a function of geometrical (thickness, pore structure, surface roughness) and intrinsic water transport (water partition and diffusion coefficients) properties of their active layers. Nevertheless, no study has evaluated the relative importance of these properties with respect to the substantial differences (i.e., > 10-fold) observed in water permeability among commercial RO membranes with active layers made of the same base material (e.g., crosslinked aromatic polyamide). Thus, we evaluated which active layer properties account for the differences observed in water permeability among five commercial RO membranes having crosslinked aromatic active layers and a wide water permeability range. We determined experimentally membrane water permeability and surface roughness, as well as active layer thickness, pore volume fraction, pore size, and water partition coefficient. We determined water diffusion coefficients in the active layers via macroscale modeling (i.e., solution-diffusion model) and microscale modeling, using as inputs to the models experimentally determined properties. Results show that the relatively large differences in water permeability among membranes were largely due to differences in water diffusion coefficients in active layers, not to differences in geometrical properties or water partition coefficients. The results from this study inform membrane developers on which active layer properties substantially change with the current membrane fabrication procedures, thus providing useful information to identify which properties could be targeted for further membrane optimization.